Embodiments of the present disclosure relate to methods and systems for estimating pressure. In particularly, but not limited to, the estimation and/or calculation of downhole pressure in wells.
Hydraulic fracturing models require knowledge of the net pressure between the fluid inside the propagating fracture and the minimum stress of the rock formation. However, this information is in general not measured during fracturing since there is no way to place a pressure gauge inside the propagating fracture.
Technically, the pressure may be measured downhole inside the treatment well (the well being used for hydraulic fracturing) in the case of water-only treatments. However for high rate sand-water fracturing a pressure gauge positioned in the treatment will be destroyed by the high-rate proppant-laden fluid (“sand-blasted”).
Where the bottomhole pressure can be estimated, the required net pressure can then be estimated from the bottomhole pressure and assumptions about the pressure drop provided by the perforation. Typically in hydraulic fracturing operations, only a memory gauge is used and so this pressure information is not available in real-time.
As such, it would be useful if downhole pressure information was available in real-time so that the fracture model could be updated and used to aid control of the treatment/hydraulic fracturing.
In vertical wells, the pressure drop in the pipe carrying fluids into the well for the high rate flowing fluid may be reasonably predicted. In which case, the bottomhole pressure may be estimated, and the required net pressure estimated from the predicted pressure drop.
However, in horizontal wells, which may undulate slightly, it is not possible to accurately predict the conditions of multiphase (sand+water+gels etc.) flow and so any estimates of pressure drop are very approximate at best. This uncertainty in pressure drop means that bottomhole pressure can only be poorly estimated; so any subsequent net pressure calculation is even more uncertain.
There is a clear need for improved estimates of the bottom hole conditions of the treatment well during pumping, particularly for high rate treatments delivered through horizontal wells.
Recent Solutions
One approach that attempts to address this need proposed by Downie et al. (Downie, R., J. Le Calvez, M. Williams, B. Dean, INTEGRATION OF FRACTURE DIAGNOSTIC TESTS AND OFFSET WELL STATIC PRESSURE WITH MICROSEISMIC EVALUATION, U.S. Patent Application Ser. No. 61/922,268) places a pressure gauge in an adjacent monitoring well, which monitoring well contains a microseismic monitoring array. Observations made with a pressure gauge in the monitoring well revealed that, in the case where there is pressure communication between the treatment well and the monitoring well (for example in a typical oil shale where thin fractured carbonates interbedded with the source shales provide the producing zones and so pressure communication between producing wells may extend thousands of feet), then such a pressure gauge will show increasing and decreasing trends associated with the net pressure. This is because pressure communication (hydraulic communication) exists in this special case.
However, there are many cases where this solution does not work, e.g. where there is low permeability, pinched layers, lenses of reservoir rock that do not have large lateral extent. In those cases improved estimates are still required.